U.S. patent application number 12/246223 was filed with the patent office on 2009-12-17 for nano-fabricated superconducting radio-frequency composites, method for producing nano-fabricated superconducting rf composites.
Invention is credited to James H. Norem, Michael J. Pellin.
Application Number | 20090312186 12/246223 |
Document ID | / |
Family ID | 41415343 |
Filed Date | 2009-12-17 |
United States Patent
Application |
20090312186 |
Kind Code |
A1 |
Norem; James H. ; et
al. |
December 17, 2009 |
NANO-FABRICATED SUPERCONDUCTING RADIO-FREQUENCY COMPOSITES, METHOD
FOR PRODUCING NANO-FABRICATED SUPERCONDUCTING RF COMPOSITES
Abstract
Superconducting rf is limited by a wide range of failure
mechanisms inherent in the typical manufacture methods. This
invention provides a method for fabricating superconducting rf
structures comprising coating the structures with single
atomic-layer thick films of alternating chemical composition. Also
provided is a cavity defining the invented laminate structure.
Inventors: |
Norem; James H.; (Downers
Grove, IL) ; Pellin; Michael J.; (Naperville,
IL) |
Correspondence
Address: |
MICHAEL J. CHERSKOV
300 NORTH STATE STREET, SUITE 5102
CHICAGO
IL
60654
US
|
Family ID: |
41415343 |
Appl. No.: |
12/246223 |
Filed: |
October 6, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60998619 |
Oct 12, 2007 |
|
|
|
Current U.S.
Class: |
505/210 ; 427/62;
505/310; 505/470 |
Current CPC
Class: |
H01L 39/14 20130101;
H01L 39/24 20130101; H01J 23/20 20130101 |
Class at
Publication: |
505/210 ;
505/470; 505/310; 427/62 |
International
Class: |
H01P 1/203 20060101
H01P001/203; H01L 39/24 20060101 H01L039/24 |
Goverment Interests
CONTRACTUAL ORIGIN OF THE INVENTION
[0002] The United States Government has rights in this invention
pursuant to contract number DE-AC02-06CH11357 between the U.S.
Department of Energy and the University of Chicago representing
Argonne National Laboratory.
Claims
1. A radio-frequency accelerator cavity adapted to accommodate an
accelerating voltage gradient between 30 MV/m and 100 MV/m, the
cavity comprising: a) a superconducting substrate defining a
conduit; b) a uniform dielectric layer covering interior surfaces
of the conduit; c) a uniform first superconducting layer covering
said dielectric layer; and d) succeeding alternating uniform
dielectric and superconducting layers overlaying said first
superconducting layer.
2. The cavity as recited in claim 1 wherein one or more of said
superconducting layers is a Type II superconductor.
3. The cavity as recited in claim 1 wherein said cavity has an
inner surface with a root mean square surface roughness of less
than 0.4 nanometers.
4. The cavity as recited in claim 1 wherein said superconducting
layers are selected from the group comprising niobium, technetium,
vanadium, alloys of niobium, alloys of vanadium, and alloys of
technetium.
5. The cavity as recited in claim 1 wherein said dielectrics are
chosen from the group consisting of Al.sub.2O.sub.3, MgB.sub.2,
TiN, Nb.sub.2O.sub.5, SiO.sub.2, HfO.sub.2 and combinations
thereof.
6. The cavity as recited in claim 1 wherein one or more of said
dielectrics and said materials are epitaxial.
7. A radio-frequency accelerator cavity adapted to accommodate an
accelerating voltage gradient between 30 MV/m and 100 MV/m, the
cavity comprising: a) a superconducting substrate defining a
conduit; b) a uniform dielectric layer covering interior surfaces
of the conduit; and c) a uniform superconducting layer covering
said dielectric layer.
8. The cavity as recited in claim 7 wherein said dielectric is
Al.sub.2O.sub.3 and said superconductor is Niobium
9. A method for producing a superconducting radio-frequency
accelerator cavity having an accelerating voltage gradient between
30 MV/m and 100 MV/m, said method comprising coating inner walls of
the cavity with one or more materials with each of said materials
deposited in one or more uniform layers that are one atom
thick.
10. The method as recited in claim 9 wherein one or more of said
materials is a dielectric.
11. The method as recited in claim 9 wherein one or more of said
materials is a superconductor.
12. The method as recited in claim 9 further comprising measuring
properties of said accelerator cavity at a high accelerating
voltage gradient and determining the coatings to be applied on the
basis of said measurements.
13. A method for producing a radio-frequency accelerator cavity
having a surface accelerating voltage gradient between 30 MV/m and
100 MV/m, said method comprising: a) subjecting a superconducting
substrate to a controlled atmosphere; b) coating the substrate with
a dielectric so as to form a uniform first layer on the substrate;
and c) uniformly coating the first layer with a superconductor so
as to form a surface with a root mean square roughness of less than
0.4 nanometer.
14. The method as recited in claim 13, said coating steps
comprising: a) contacting walls of said cavity with a fluid
containing a first precursor chemical of the coating material so as
to facilitate attachment of the first precursor to the walls; b)
purging excess first precursor molecules from the cavity, with the
purging facilitated with a non-reacting gas such as nitrogen,
helium, neon, or argon; c) contacting the first precursor contacted
walls with a fluid containing a second precursor chemical such that
the second precursor reacts with the first precursor chemical to
form a single-molecule layer of the predetermined material on the
wall of the chamber; and d) purging all remaining matter from the
cavity.
15. The method as recited in claim 14 wherein said steps of a)
contacting walls of said cavity with a fluid containing a first
precursor chemical of the coating material so as to facilitate
attachment of the first precursor to the walls; b) purging excess
first precursor molecules from the cavity, the purging facilitated
with a non-reacting gas such as nitrogen, helium, neon, or argon;
c) contacting the first precursor contacted walls with a fluid
containing a second precursor chemical such that the second
precursor reacts with the first precursor chemical to form a
single-molecule layer of the predetermined material on the wall of
the chamber; and d) purging all remaining matter from the cavity
are repeated in sequence until a desired thickness of deposited
materials is achieved.
16. The method as recited in claim 13 wherein a one nanometer
thick, hole-free coating is applied within in 30 seconds.
17. The method as recited in claim 13 wherein said controlled
atmosphere is at a temperature of between 26.degree. C. and
200.degree. C.
18. The method as recited in claim 13 wherein said dielectric layer
is between 3 and 100 nm thick.
19. The method as recited in claim 13 wherein said dielectric is
coated with a superconductor between 3 and 100 nm thick.
20. The method as recited in claim 13 wherein one or more of said
superconductors and dielectrics are epitaxial.
21. The method as recited in claim 13 wherein said superconductors
and dielectrics are deposited one atomic layer at a time.
22. The cavity as recited in claim 1 wherein said substrate is
coated with layered structures with alternating dielectric and
superconductor layers chosen from the group consisting of Nb,
Al.sub.2O.sub.3, MgB.sub.2, TiN, NbN and combinations thereof.
Description
[0001] This utility application claims the benefits of U.S.
Provisional Application No. 60/998,619 filed on Oct. 12, 2007.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] This invention relates to an improved method for preparing
radio-frequency ("rf") accelerator cavities, and more particularly,
this invention relates to an improved method for preparing high
voltage-gradient superconducting rf cavities through controlled
deposition of dielectric and superconducting materials.
[0005] 2. Background of the Invention
[0006] Superconductors are metals that exhibit vanishing resistance
at very low temperatures, i.e. temperatures a few degrees above
absolute zero (0 degrees Kelvin). Typically, such temperatures are
achieved by placing the metal in thermal contact with liquid
Helium. Superconducting radio-frequency ("SCRF") cavities have been
adopted throughout the world for the acceleration of particle
beams, but SCRF cavities have not been able to routinely reach
theoretically expected performance for a variety of reasons. As a
result, accelerator designs have been increasing in complexity,
cost, and length over the past decades, due to the inability to
increase the accelerating voltage gradient of superconducting
cavities over 30 MV/m (Million Volts/meter). The International
Linear Collider (ILC) design, which is currently estimated to be
about 20-25 miles long, is based on the limits of today's
technology.
[0007] As shown in FIG. 1, a typical SCRF cavity designated as
numeral 10 resembles a bellows conduit having alternations of
constrictions 15 and recesses 19. An rf voltage generator induces
an electric field inside the cavity such that electrons injected at
an end 16 of the cavity are accelerated towards the other end 17.
Typically the generator frequency is 1.3 GHz. Fabricated from heavy
sheet metals, SCRT cavities have many surface imperfections and
oxide layers, even after extensive cleaning procedures. As the
regenerated current is confined within roughly the first 100
nanometers of the cavity's surface, the main sources of cavity
failure are these surface imperfections. Imperfections include
protrusions (referred to herein as asperities), burrs, ridges,
scratches, and other defects.
[0008] The inventors have found that high local fields result from
small local radii of the imperfections. Where local radii of the
asperities are larger, the local fields are smaller, and the
cavities do not fail. Approximately, the local field E is inversely
proportional to the local radius, thus E.about.1/r.
[0009] The largest electric fields in a SCRF cavity are found near
the constrictions 15 and the largest magnetic fields have circular
field lines at the recesses 19 in planes perpendicular to the
cavity axis .alpha..
[0010] A SCRF cavity is first formed from superconducting material,
such as niobium. Then, the inside of the structure is
electropolished, cleaned and treated in a variety of ways. Finally,
the structure is rinsed with high-pressure water. The structures so
fabricated accommodate a maximum field of 30 MV/m at 1.3 GHz.
[0011] These and other typical superconducting structures fail as a
result of a number of mechanisms, for example: 1) field emission
(in which free electrons circulating in the highly conductive metal
get pulled out of the metal's surface, generating discharges that
can "short out" or overheat the structure), 2) quench fields, where
the magnetic field exceeds the maximum field that the
superconductor can support, 3) high field Q slope, where losses
(due to magnetic oxides) degrade the ability of the cavity to store
energy, 4) "multipactor", where resonant amplification of parasitic
currents is caused by surface properties, 5) thermal effects
(circulating current-heating of the material, thus causing stresses
and deformation), 6) breakdown, where arcs are produced, 7) power
and cryogenic load limits, 8) assembly defects and particulate
generation, 9) Lorentz forces, where internal fields distort the
structure, 10) microphonics, where external acoustic noise distorts
the structure, and 11) local heating of hot spots.
[0012] Cavities fabricated from ordinary metals, such as copper,
silver, stainless steel, can also fail at comparable gradients.
[0013] It is important that all of the above failure mechanisms be
addressed in the search for higher gradient SCRF cavities.
[0014] A need exists in the art for a method to increase the energy
gradient of SCRF accelerator cavities over the current limit of 30
MV/m. The method should provide economic benefits by making the ILC
and other accelerators shorter, more power efficient, capable of
reaching higher energies and by reducing construction and operating
costs. The method should also enable the production of "table top"
accelerators and smaller, high-gradient cavities compared to the
size of cavities now used.
SUMMARY OF THE INVENTION
[0015] It is an object of the present invention to provide a method
for the preparation of accelerator superconducting cavities that
overcomes many of the disadvantages of the prior art.
[0016] A further object of the present invention is to provide a
method for producing accelerator superconducting cavities with
gradients as high as 100 MV/m. A feature of the invention is the
serial application of one or several materials on an underlying
substrate, whereby each of the layers is uniformly one molecule or
atom thick. An advantage of the invention is the elimination of the
elaborate cleaning and etching processes currently required when
producing rf cavity surfaces. Another advantage of this invention
is its use in situ, (i.e., when the cavity is already incorporated
in an accelerator) so as to avoid contamination, and/or as part of
ongoing maintenance.
[0017] Another object of the present invention is to provide
superconducting accelerator cavities which are 50 percent shorter
than typical structures producing similar acceleration. A feature
of the invention covering a superconducting underlayment with
layers of insulating material arranged in alternating
configurations with layers of superconducting material. An
advantage of the invention is that the insulating materials shield
the superconducting underlayment from quenching magnetic fields
while simultaneously directing oxygen from the original surface of
the underlayment and into the bulk of the underlayment.
[0018] Briefly, the invention provides a radio-frequency
accelerator cavity adapted to accommodate an accelerating voltage
gradient between 30 MV/m and 100 MV/m, the cavity comprising a
superconducting substrate defining a conduit; a dielectric layer
covering interior surfaces of the conduit; a first superconducting
layer covering said dielectric layer; and succeeding alternating
dielectric and superconducting layers overlaying said first
superconducting layer.
[0019] Also provided is a radio-frequency accelerator cavity
adapted to accommodate an accelerating voltage gradient between 30
MV/m and 100 MV/m, the cavity comprising a superconducting
substrate defining a conduit; a dielectric layer covering interior
surfaces of the conduit; and a superconducting layer covering said
dielectric layer.
[0020] The invention also provides a method for producing a
superconducting radio-frequency accelerator cavity having a surface
accelerating voltage gradient between 30 MV/m and 100 MV/m, the
method comprising coating inner walls of the cavity with one or
more materials with each of said materials deposited in one or more
layers that are one atom thick. One or more of said materials may
be chosen to be a superconductor and one or more of said materials
may be chosen to be a dielectric.
[0021] The invention further provides a method for producing a
radio-frequency accelerator cavity having a surface accelerating
voltage gradient between 30 MV/m and 100 MV/m, said method
comprising subjecting a superconducting substrate to a controlled
atmosphere; coating the substrate with a dielectric so as to form a
first layer on the substrate; and coating the first layer with a
superconductor so as to form a surface having a root mean square
roughness of less than 0.4 nanometer.
[0022] Specifically the present invention provides a method for
producing a radio-frequency accelerator cavity having an
accelerating voltage gradient between 30 MV/m and 100 MV/m., said
method comprising reducing the following defects by the following
means: [0023] a) field emission, by conformal coating of the cavity
to reduce local fields; [0024] b) quench fields, where the magnetic
field exceeds the maximum field that the superconductor can
support, by depositing a plurality of monoatomic layers of
predetermined materials; [0025] c) high field Q slope, where the
ability of the cavity to store energy is degraded, by controlling
the chemical nature of the cavity surface; [0026] d) resonant
amplification of parasitic currents caused by surface properties,
by controlling the chemical nature of the cavity surface; [0027] e)
heating of the material because of the circulating currents, by
controlling the chemical nature of the cavity surface; [0028] f)
voltage breakdown, where arcs are produced, by conformal coating of
the cavity to reduce local fields; [0029] g) power and cryogenic
load limits, by controlling the chemical nature of the cavity
surface; [0030] h) assembly defects and particulate generation, by
in situ treatment of the cavity surface; [0031] i) internal fields
distorting the cavity, by applying pre-determined coatings to the
cavity surface that the cavity can be made more rigid than bulk
niobium; [0032] j) external acoustic noise distorting the cavity,
by applying pre-determined coatings to the cavity surface so that
the cavity surface can be made more rigid than bulk niobium; and
[0033] k) local heating of the cavity hot spots, by controlling the
chemical nature of the cavity surface;
[0034] These coatings are applied so as to be uniform on an atomic
scale and of a thickness approximately equal to the measured
typical size of surface imperfections.
BRIEF DESCRIPTION OF THE DRAWING
[0035] The invention together with the above and other objects and
advantages will be best understood from the following detailed
description of the preferred embodiment of the invention shown in
the accompanying drawing, wherein:
[0036] FIG. 1 is a schematic diagram of cross section of a surface
of a radio-frequency accelerator superconducting cavity (SCRF);
[0037] FIG. 2 is a schematic depiction of the results of prior art
methods for producing superconducting radio-frequency (SCRF)
cavities;
[0038] FIG. 3 is a schematic depiction of the results of the
invented method for producing superconducting radio-frequency
(SCRF) cavities, in accordance with features of the present
invention; and
[0039] FIG. 4 is a schematic depiction of the coating of a surface
imperfection by means of the invented method for producing
superconducting radio-frequency (SCRF) cavities, in accordance with
features of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0040] The present invention provides a radio-frequency accelerator
cavity adapted to receive an accelerating voltage gradient between
30 MV/m and 100 MV/m. The cavity defines an interior surface or
wall coated with one or more predetermined materials which minimize
imperfections of the surface, so that the inner surface has a root
mean square surface roughness of less than 0.4 nanometers.
[0041] The present invention also provides a method for producing a
radio-frequency accelerator cavity having an accelerating voltage
gradient between 30 MV/m and 100 MV/m. The method comprises coating
said wall with one or more predetermined materials.
[0042] Once installed, these accelerator cavities must retain the
same configuration to within one part in ten billion. A special
advantage of the invented process is that if SCRF cavities are
degraded through long-time use in an accelerator, the cavities
could be repaired in situ without dismantling the accelerator and
without bringing the cavities to atmospheric pressure. (For
example, the accelerator can be built with appropriate attachments
for application of the invented coating process).
[0043] A myriad of deposition means may be used, including but not
limited to cluster ion beam (GCIB) smoothing, Dry Ice Cleaning
(DIC), Electron Cyclotron Resonance (ECR) plasmas, and Atomic Layer
Deposition (ALD).
[0044] ALD use in the instant protocol enables application of
pin-hole free (i.e. continuous) coatings (as verified by
measurements of electrical resistance of thin films with large
surface areas) The coatings attain one nanometer thickness within a
few seconds (i.e. between 1 and 5 seconds) after deposition
begins.
[0045] Incorporating ALD in the instant protocol facilitates
elimination of the aforementioned (a) through (k) eleven (11)
failure modes of typically-fabricated cavities. For example,
conformed coatings reduce local fields while exact control of the
surface chemistries eliminates Q-slope, multipactor, load limit and
local heating problems. In-situ deposition enabled by AFD allows
for the formation of coatings after assembly thereby minimizing
assembly defects, Lorentz effects, and acoustic noise problems.
Foundation Substrate and Overlaying Layer Detail
[0046] The present invention utilizes a plurality of sub-micron
thick layers (in effect, laminates) which overlay superconductors,
such as Type II elemental superconductors niobium, vanadium and
technetium. Type II superconductors are preferable because they
remain superconducting at higher ambient temperatures and magnetic
fields. Alloys such as niobium tin, niobium titanium, niobium
germanium, and niobium silicon and superconductors are also
suitable as the superconductor foundation substrate.
[0047] In general, the substrate material should be able to reach
the highest field possible without losses, defined by the lower
critical field, H.sub.c1. The materials used in the layers should
be able to reach the highest fields obtainable, defined by
H.sub.c2. (H.sub.c1 and H.sub.c2 are defined as follows: When the
magnetic field is below H.sub.c1, no magnetic fields can penetrate
in the superconductor. Above H.sub.c1 and below H.sub.c2, some
magnetic field can penetrate in the superconductor. Above H.sub.c2,
the material loses its superconductivity).
[0048] Deposition of layers on top of the foundation substrate is
facilitated with the use of commercial grade pure gases, to ensure
that no contamination is present. An exemplary embodiment is
depicted in FIG. 3, which shows a typical superposition of
laminates deposited on a substrate 30 of bulk superconductor. In
one illustrative embodiment, the laminate structure comprises a
layer 43 of active Niobium, a layer 45 of MgB.sub.2, where the
MgB.sub.2 screens or otherwise isolates the substrate from the
alternating magnetic fields, a multipactor suppression layer 47 of
TiN, and insulator layers 41 that stop diffusion of ions (including
hydrogen, oxygen, and nitrogen) from one laminate into another.
[0049] Any superconducting material with a higher H.sub.c2 greater
than Niobium is suitable as a means for isolating the underlying
substrate from alternating magnetic fields. As such, compounds
including, but not limited to, MgB.sub.2, NB.sub.3Sn, V.sub.3Si,
Nb.sub.3Ge, Nb.sub.3Si and combinations thereof are such
materials.
[0050] Any compound with a secondary emission coefficient less than
one (i.e. producing fewer electrons tan are absorbed) is a suitable
multipactor suppression agent. As such, compounds including, but
not limited to, TiN, Nb.sub.2O.sub.5, In.sub.2O.sub.3, SnO.sub.2,
ZnO and combinations thereof are suitable candidates.
[0051] The invented insulating/superconducting layers combination,
and method for producing same, provides a means for protecting
(filtering) the underlying foundation substrate from high rf
magnetic fields that would otherwise quench the superconductor at a
field of about 0.2 Tesla. Instead, the collective effect of the
invented layer combination provides insulation up to 1 T.
[0052] FIG. 3 also illustrates another embodiment of the invention
where a substrate 30 defines a superconducting surface 31. A
plurality of layers 43, 45, 47 of a high-temperature superconductor
such as NbN, or NB.sub.3Sn overlay the foundation surface 31. An
Iron Nictide (Ba.sub.1-xK.sub.xFe.sub.2As.sub.2) may also be used
for that purpose. Intermediate the layers 43, 45, 47 are positioned
one or more layers 41 of an insulator.
[0053] A myriad of dielectric materials can comprise the insulating
layer, including, but not limited to, Al.sub.2O.sub.3,
Nb.sub.2O.sub.5, SiO.sub.2, HfO.sub.2 and combinations thereof.
[0054] The ALD process conformally coats asperities and other
features of the foundation surface 31. The coatings over these
features comprises uniform, well characterized material with known
properties (insulator, conductor, superconductor), Also, the
coatings provide a means for smoothing the system, akin to snow on
a rock pile, so as to make the surface "nanosmooth" (i.e. smooth on
a scale of a few nanometers), pure, homogenous, continuous and
insulating.
[0055] The coatings provide a means for smoothing-out the
projections so as to transform the small radii of the projections
to large radii (concomitantly producing low fields), to make them
failure proof. The inventors found that many of the asperites are
semi-spherical and have radii of from 10 nm to 30 nm. Generally,
transforming asperities to define radii greater than 100 nm (i.e.,
3 to 10 times the original asperite radii size) will produce the
desired low field effect. Low fields are those less than 10.sup.9
V/m. Smoothing out of asperities and other features typically
requires coating thicknesses of coatings in the 20-100 nm
range.
[0056] The precise nature of the invented deposition configuration
makes it able to simultaneously attack all of the aforementioned
failure modes seen in prior art systems. Also it allows coating
after assembly. Furthermore, the resulting constructs exhibits
lower regenerated heating and better thermal properties.
[0057] The inventors have measured the diameters of the asperities
that cause field emission and breakdown events in accelerator
cavities and found that they are on the order of 10-100 nm,
prevalently in the 20 to 60 nm range, with most of the field
emission being produced by asperities with a diameter of less than
100 nm. As such, optimal coating parameters have been deduced from
these measurements. Details of these measurements are found in J.
Norem et al, "Dark current, breakdown, and magnetic field effects
in a multicell, 805 MHz cavity." Physical Rev. Special
Topics--Accelerators and Beams, Vol. 6, 072001 (2003), incorporated
herein by reference. Typical height "h" of an asperity is
approximately equal to or smaller than its diameter "d", i.e
between 20 and 60 nm, and rarely exceeds 100 nm. High field
conditioning of a cavity decreases the number of asperities and
their size.
[0058] FIG. 4 is a schematic of the invented deposition process
where in a single layer 49 is deposited on a substrate 30 that
defines a superconducting surface 31. For instance, the layer 49
may be a coating of Niobium on a substrate of Niobium, where the
coating is applied to smooth out asperities or contaminants such as
the asperity 50 of height h and diameter d. (Typically d and h are
equal to each other within a factor of two). The coating layer has
a thickness t that is approximately equal to the height h or
greater than h. Generally, the tip 51 of the asperity 50 has a
smaller radius of curvature than the portion 52 of the over-coating
positioned above the tip 51. This portion has a radius of curvature
approximately equal to t.
[0059] Inasmuch as the local field E at each of these tips is
inversely proportional to the radius of curvature, the deposition
of the layer 49 results in a much smaller local field E. This
smaller local field E allows the accelerator cavity to sustain a
much larger accelerating voltage gradient.
[0060] Generally, good results are obtained as long as the coating
49 covers the asperity 50. Preferably, a coating thickness t of
between h/10 to h is used. Most preferable configurations are with
t equal to or greater than h/2, such that a coating exists over the
tip 51 of the asperity that is as thick as h/2. Given that
asperities have heights in the 20 to 100 nm range, a coating of 20
to 100 nm thickness allows a high accelerating voltage
gradient.
[0061] In fact, one can first measure the surface properties of an
accelerator cavity using methods such as those described in J.
Norem et al. (see supra) and from these measurements determine the
materials and thicknesses to be deposited.
[0062] A salient feature of this invention is that the deposits can
be multi-layered, so as to filter quench fields. The deposits are
pure enough so that high field "Q slope" effects (due to scattering
from magnetic oxide contaminations near the surface) are avoided
and the ability of the cavity to store energy is maintained. The
deposited layers can be covered with monolayers of a film (e.g. dry
oxide, TiN) having much lower secondary emission coefficients to
prevent the occurrence of multipacting and other effects.
[0063] Generally, insulator layers are positioned between the
superconducting surface. This arrangement shields the primary
current carrying elements from quench fields.
[0064] Conformal characteristics of the coatings produced by the
method enable in-situ deposition in existing structures, both to
minimize assembly defects and to repair defects. The interiors of
long narrow tubes have already been coated. The ratio length to
diameter of these tubes is about 5,000 LTD (length to diameter),
comparable to the (underperforming) SNS linear accelerator
("liniac").
Deposition Detail
[0065] The invented process enables deposition rates on the order
of one micron (micrometer, .mu.m) per hour. Moreover it is a
parallel, non-line-of-sight, conformal coating technique. This
technique allows the growth of films on complex cavity structures.
The materials are covalently bonded to the surface substrate and
other layers. ALD is unique in that it allows the deposition of a
layer that is uniform on an atomic scale covering the entire inner
surface of a structure.
[0066] Thickness control to the single-atomic-layer level is easily
achieved with ALD. It is similar in chemistry to chemical vapor
deposition (CVD), except that the ALD reaction breaks the CVD
reaction into two half-reactions, keeping precursor materials
separate during the reaction. ALD film growth on the sc cavity
underlayment is self-limiting and based on surface reactions, which
makes achieving atomic-scale deposition control possible. By
keeping the precursors separate throughout the coating process,
atomic layer control of film growth as fine as -0.1 angstroms per
monolayer can be obtained. The coating is applied to all exposed
portions of the cavity surfaces, so that there is no field
penetration at the corners or edges of the cavity.
[0067] The invented protocol can be performed at or near room
temperature (i.e. from 100.degree. C. to 250.degree. C.). The
temperature at which ALD is performed depends on the specific
reactions that are utilized, with reaction rates changing by
roughly a factor of 2 for every 10 degrees Fahrenheit. Preferably,
the depositions occur at roughly 100-200 degrees Celsius with the
underlying cavity structure heated prior to deposition. Capping or
insulating layers are be applied to protect the final surface
(i.e., surface exposed to the accelerated plasma) from impurities
coming from the surface of the bulk material. Only the top
monolayers (from 100 nm to 1000 Angstroms) of the resulting
construct participate in the gradient. The surface is pin-hole free
and can be reproduced precisely.
Protocol Detail
[0068] An embodiment of the invented method for coating a
predetermined material onto the inner walls of a chamber
comprises:
[0069] a) contacting the walls with a fluid at the same temperature
as the walls that contains a first precursor chemical of the
coating material so as to facilitate attachment of the first
precursor to the walls;
[0070] b) purging excess first precursor molecules from the
chamber, the purging facilitated with a non-reacting gas such as
nitrogen, helium, neon, or argon;
[0071] c) contacting the first precursor contacted walls with a
fluid at the same temperature as the walls that contains a second
precursor chemical such that the second precursor reacts with the
first precursor chemical to form a single-molecule layer of the
predetermined material on the wall of the chamber; and
[0072] d) purging all remaining matter from the cavity.
[0073] Steps (a) through (d) are repeated in sequence until a
desired thickness of deposited materials is achieved.
[0074] Typically, the cavity is evacuated before step (a) and the
fluid is a gas at a pressure of between 0.0001 and 0.01
atmospheres, and preferably at approximately 0.001 atmospheres.
Insulator Layer Detail
[0075] To make an insulator layer of alumina, Al.sub.2O.sub.3, a
precursor chemical such as trimethylaluminum (TMA) is injected into
the system and for a time and at a temperature to allow the TMA
molecules bond to the substrate. Excess molecules are purged with a
nonreacting gas, such as, but not limited to, nitrogen or noble
gases (e.g. He, Ar, Ne). Next, water vapor is injected and reacts
with the TMAI to form a single-molecule layer of Al.sub.2O.sub.3 on
the substrate while CH.sub.4 (methane) gas is released. The latter
is then purged from the chamber with a nonreacting gas (as defined
supra). The process is then repeated until a pre-determined
thickness is achieved.
[0076] In practice the reacting and nonreacting gasses are
evacuated from one end of the reactor tube to enable a continuous
flow of gas over surfaces to be coated.
[0077] The inventors' experiments have shown that thin (3-10 nm)
layers Al.sub.2O.sub.3 are impervious to air. The TMAI exposures
for Al.sub.2O.sub.3 in the ALD process tend to chemically reduce Nb
oxide given that the Aluminum extracts the oxygen from the niobium
due to the former being more active. One can further heat treat Nb
at a higher temperature (500-800.degree. C.) and modify the niobium
oxide.
[0078] Oxygen atoms that have not reacted with aluminum diffuse
into the bulk of the niobium, and the alumina prevents oxygen
penetrating the superconducting layers from the air. A variety of
additional coatings can be applied to insure that secondary
emission coefficients are below unity. Thus, the capping film
reduces two different types of usual SCRF problems. As alumina
surfaces may be subject to multipacting, another layer of
superconducting material on the Al.sub.2O.sub.3 layer can be
applied. Annealing of a Al.sub.2O.sub.3 capped surface at a
temperature of approx. 500.degree. C. (a form of high temperature
annealing) decreases the incidence of magnetic scattering in the
medium.
Example
[0079] An embodiment of the present invention is a coating layer of
alumina, Al.sub.2O.sub.3, several molecules thick to cover a
niobium surface with a chemically impermeable insulator. Next a
layer of niobium fluoride is deposited on the alumina surface, and
then another layer of alumina is deposited to protect the niobium
from oxidation. Alumina is a good thermal conductor, a good
electrical conductor, and resists oxidation. The first layer of
alumina will permit any oxygen attached to the niobium substrate to
migrate into the interior of the substrate and mitigate any decline
in material properties. The surface with a combination of
alumina/niobium fluoride/alumina on bulk niobium enables the
resulting construct to operate at significantly higher fields than
have been seen in any existing rf structure.
[0080] Other materials, such as magnesium diboride as a
superconducting layer, and additional layers of alumina, are added
to provide different properties. The inventors found that vortexes,
which increase the losses in the superconducting path, are
eliminated when the layers of superconductor are too thin to stably
maintain them.
[0081] The quality of the films is monitored with measurements
using ellipsometry, x-ray photoelectron spectroscopy, Atom Probe
Tomography, SIMS, point contact tunneling, and other techniques.
The inventors have conducted experiments showing that the presence
of magnetic oxides can contribute to superconducting loss mechanism
and, using point contact tunneling, they have shown the thermal
dependence of this loss.
[0082] The inventors have conducted extensive tests of the
superconducting properties of ALD coatings using point contact
tunneling. The testing of properties of ALD coatings of single rf
cells comprises rf measurements of a cell before and after coating.
In addition to the tests on complete cells, point contact tunneling
measurements determine the properties of the superconductors at the
interface between the bulk niobium and the oxide layer.
[0083] Tests have experimentally verified that ALD can grow
coatings of alumina which are 10 molecules thick on superconducting
niobium. Such layers have been shown to protect the niobium from
oxidation in air, and in fact, reduce the niobium concentration on
the surface. Heating this composite at 450.degree. C. drives any
oxygen present on the surface into the bulk material, thereby
producing a clean niobium/alumina interface. Subsequent tests have
shown that it is possible to overcoat the alumina with titanium
nitride, which would permit the composite to be used in high power
tests without multipactoring (parasitic secondary electron
emission).
[0084] Thus, while superconducting rf cavities were limited
heretofore by a wide range of failure mechanisms, synthesizing
films to overlay superconducting surfaces cures the above
defects.
[0085] It is to be understood that the above description is
intended to be illustrative, and not restrictive. For example, the
above-described embodiments (and/or aspects thereof) may be used in
combination with each other. In addition, many modifications may be
made to adapt a particular situation or material to the teachings
of the invention without departing from its scope. While the
dimensions and types of materials described herein are intended to
define the parameters of the invention, they are by no means
limiting, but are instead are exemplary embodiments. Many other
embodiments will be apparent to those of skill in the art upon
reviewing the above description. The scope of the invention should,
therefore, be determined with reference to the appended claims,
along with the full scope of equivalents to which such claims are
entitled. In the appended claims, the terms "including" and "in
which" are used as the plain-English equivalents of the terms
"comprising" and "wherein." Moreover, in the following claims, the
terms "first," "second," and "third," are used merely as labels,
and are not intended to impose numerical requirements on their
objects. Further, the limitations of the following claims are not
written in means-plus-function format and are not intended to be
interpreted based on 35 U.S.C. .sctn. 112, sixth paragraph, unless
and until such claim limitations expressly use the phrase "means
for" followed by a statement of function void of further
structure.
* * * * *